Method for encapsulating an integrated semi-conductor circuit

ABSTRACT

Method for encapsulating an integrated semi-conductor circuit (die) comprising the following steps: 
     a) mounting the semi-conductor circuit onto the surface of a so-called lead frame, 
     b) attaching connecting wires between the contact surfaces of the semi-conductor circuit and selected parts of the lead frame (bonding operation), 
     c) by means of a mould producing a plastic housing which at least encapsulates the semi-conductor circuit, the supporting surface, the bonding wires and part of the lead frame, 
     d) the mould comprises an inwards extending section of which the end surface in the closed situation of the mould extends parallel to the free upper side of the integrated semi-conductor circuit at short distance thereof, and 
     e) before closing the mould a layer of heat resistant deformable material in the form of a ring or a continuous layer is brought in between the upper side of the integrated semi-conductor circuit and said end surface of the inwards extending part, which layer not or hardly adheres to the plastic housing.

BACKGROUND OF THE INVENTION

The invention relates to a method for encapsulating an integratedsemi-conductor circuit (die) comprising the following steps:

a) mounting the semi-conductor circuit onto the surface of a so-calledlead frame,

b) attaching connecting wires between the contact surfaces of thesemi-conductor circuit and selected parts of the lead frame (bondingoperation),

c) by means of a mould producing a plastic housing which at leastencapsulates the semi-conductor circuit, the support surface, thebonding wires and part of the lead frame.

Such a method is generally known and is applied on a wide scale forencapsulating integrated semi-conductor circuits. In general the createdhousing will encapsulate the semi-conductor circuit completely such thatonly the connecting pins, forming part of the so-called lead frame, areextending outside the housing whereby the semi-conductor circuit as suchis therefore screened from outer influences as much as possible.

In contrast therewith the invention is especially directed to a methodfor obtaining an encapsulation around an integrated semi-conductorcircuit which comprises opto-electronic components whereby the housingshould have a cavity which gives an open connection between the outsideworld and the active area on the `die`. If this cavity should be closed,it should be closed with window, opaque for radiation. Within themeaning of the invention radiation is both radiation in the visible partof the spectrum as well as radiation infrared or ultraviolet part of thespectrum.

The U.S. Pat. No. 5,200,367 comprises on the one hand a description ofso-called ceramic housings and comprises on the other a description of aplastic housing which could be used as a replacement for the ceramichousing. In both cases however the housing is embodied such that theintegrated circuit at the side, where the radiation should impinge onthe circuit, is open. This open section can be covered by a separateplate of glass or, in case the semi-conductor circuit does not compriseany light sensitive components, be covered by a plate of anothermaterial such as a metal plate.

It is known for the expert in this field that the use of ceramichousings will lead to increase of costs of the final electroniccomponent. Ceramic housings are therefore almost exclusively used forcomponents which have to fulfil high requirements.

The steps, which successively have to be carried out according to themethod described in U.S. Pat. No. 5,200,367 to obtain the desired endresult are in their sequence clearly different from the generally usedsequence. As is indicated shortly in the first paragraph, it is known toinstall first of all the chip or die on a lead frame and to attach thebonding wires between the chip or die and selected parts of the leadframe. Thereafter this intermediate product is encapsulated using amould. This last step does not have to be carried out in a directsequence with the earlier steps and can be performed at anotherlocation.

According to U.S. Pat. No. 5,200,367 first of all the lead frame ispositioned within the mould and a housing is formed which is open at theupper side. Thereafter, the chip is installed on the related not coveredsection of the lead frame followed by bonding the connecting wiresbetween the chip and selected sections of the lead frame (which for thatpurpose are not encapsulated in the housing). Both mentioned processeshave to be carried out in a cavity in the already made housing. Becauseof that the number of possibilities to attach the chip to the lead frameis restricted. Welding at higher temperatures for instance willcertainly lead to housing damage. Also the presence of the housing assuch may for a hindrance for applying the usual bonding machines forattaching the bonding wires.

SUMMARY OF THE INVENTION

The purpose of the invention is now to indicate in which way anintegrated semi-conductor can be brought into a plastic housingcomprising a cavity using as much as possible method steps in the usualorder.

In agreement with the object of the method according to the invention isnow characterized in that:

d) the mould comprises an inwards extending section of which the endsurface in the closed situation of the mould extends parallel to thefree upper side of the integrated semi-conductor circuit at shortdistance thereof, and

e) before closing the mould a layer or ring of heat resistant deformablematerial is brought in between the upper side of the integratedsemi-conductor circuit and the end surface of the inwards extendingpart, which layer not or hardly adheres to the plastic housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to theattached drawings.

FIG. 1 illustrates a cross-section through a mould with therein thesemi-conductor circuit, the lead frame and the connecting wires betweenthe circuit and the frame.

FIG. 2 illustrates a cross-section through the mould after the openingat the upper side is closed by an inwards extending component, fluidmaterial is applied in the form of a layer or ring between the componentand the semi-conductor circuit and the plastic, which forms theencapsulation, is injected.

FIG. 3 illustrates a cross-section through the housing after removingthe housing from the mould.

FIG. 4 illustrates a cross-section through the housing after positioninga closing window.

DETAILED DESCRIPTION OF THE EMBODIMENTS

During the manufacturing of integrated semi-conductor circuits ingeneral a large number of these circuits are made simultaneously ontoone single large silicon platelet, a so-called wafer. After completingthe actual integration process the wafer is divided into the separatedsemi-conductor circuits or chips using known techniques, for instance bycutting or etching. (In English literature these chip are also called"die" or "pellet"). Thereafter, each chip is positioned on a metalframework comprising contact pins which are mutually connected throughconnecting parts such, that as a whole they form a so-called lead frame.The central section of such a lead frame comprises a mounting surfaceonto which the chip can be positioned and attached by means of solderingor another method which is known as such. After fixing the chip in thismanner onto the central section of the lead frame the bonding wires areattached between the various contact pins and the connecting surfaces or"pellets" on the chip. The result at the end of this operation isschematically shown in cross-section in FIG. 1.

In FIG. 1 the contact pins are indicated by 10a and 10c, whereas thecentral mounting surface of the lead frame is indicated by 10b. Ontothis mounting surface 10b (in the English language called a die pad) thechip 12 is fixed in a known manner, which does not play a role withinthe scope of the invention. Furthermore, between the connecting pins 10aand 10c on the one hand and the chip 12 on the other hand bonding wires14a and 14b are attached. All the used techniques therefore are known assuch as do not require further explanation.

In the illustrated embodiment the mould comprises two parts, i.e. abottom part 16 and a cover part 18. In contrast with the usuallycompletely closed moulds an opening 20 is made in the cover part 18above the semi-conductor circuit 12.

A further intermediate step in the method is illustrated in FIG. 2. Theopening 20 is closed by means of inwards extending part 22 of which thecross-sectional shape corresponds with the shape of the opening 20. Inthe illustrated embodiment this part comprises a central column 22a anda ring 22b, fitting about the column. The column 22a extends furtherinside than the ring 22b for reasons which will be made clearhereinafter.

Preceding the insertion of the part 22 the under side thereof,especially the under surface of column 22a, is applied with a certainamount of heat resistant deformable material 23 in the form of a ring ora continuous layer, the material being of a type which not or hardlyadheres to the plastic which hereafter will form the housing. The amountof this material is selected such that after locating the part 22 asection of the semi-conductor circuit surface is covered or enclosed bythis material as is illustrated in FIG. 2. Of course, as an alternativethe deformable material can be applied to the surface of thesemi-conductor circuit whereafter the part 22 is brought in position.However, the first method is preferred because of the very simplecorrect positioning of the material.

Preferably the deformable material consists of a gel, especially asilicon gel. This material does not or hardly adhere to the varioustypes of epoxy which in general are used for making the actualencapsulation.

Thereafter the epoxy or resin is injected in the free space in the mouldto create the encapsulation. The resin is indicated by 24 in FIG. 2.

After at least partly hardening of the now encapsulated semi-conductorcircuit, removing it out of the mould and removing the deformablematerial 23 the intermediate product will be obtained which is shown incross-section in FIG. 3. The semi-conductor circuit is partlyencapsulated and a part of the upper surface of the semi-conductorcircuit is still left free. The opening above this section has astepwise shape as is clearly visible in FIG. 3. This shape, resultingfrom the use of a column 22a and a separate tube 22b is suitable formounting a window 26 in the way as illustrated in FIG. 4. This windowcan be attached for instance by using a suitable adhesive 28.

It will be clear that the presented method can be applied to make corecavities above one semi-conductor circuit. In that case the mould shouldcomprise a corresponding number of openings, each of which has to beclosed by an inwards extending element, such as the combination of acolumn 22a and the tube 22b.

Furthermore, it will be clear that instead of this combination onesingle component of adapted shape can be applied.

Also it will be clear that the position of the open cavity can be variedbased on the location of the active area on the `die`. In that case themould should comprise a corresponding opening 20 the position of whichhas to be closed by an inwards extending element.

I claim:
 1. A method for encapsulating an integrated semi-conductorcircuit (die) comprising the following steps:a) mounting thesemi-conductor circuit onto the surface of a lead frame, b) attachingconnecting wires between the contact surfaces of the semi-conductorcircuit and selected parts of the lead frame (bonding operation), c) bymeans of a mould producing a plastic housing which at least encapsulatesthe semi-conductor circuit, the supporting surface, the bonding wiresand part of the lead frame,wherein d) the mould comprises an inwardsextending section of which the end surface in the closed situation ofthe mould extends parallel to the free upper side of the integratedsemi-conductor circuit at short distance thereof, and e) before closingthe mould a layer of heat resistant deformable material in the form of aring or a continuous layer is brought in between the upper side of theintegrated semi-conductor circuit and said end surface of the inwardsextending part, which layer not or hardly adheres to the plastichousing.
 2. A method according to claim 1, wherein the material isapplied to end surface of the inwards extending part such that the endsurface is covered or enclosed by said material.
 3. A method accordingto claim 1, wherein said material consists of a silicon gel.
 4. A mouldfor applying a method according to claim 1, comprising two or moredetachable components together defining a space corresponding to thedesired outer shape of the plastic housing, wherein said inwardsextending part consists of a separate columnar component which,preceding the use of the mould, is attached in a thereto destinedopening in the wall of one of the other components.
 5. A mould accordingto claim 4, wherein the inwards extending part of the separate columnarcomponent comprises said end surface as well as a columnar wallperpendicular thereto.
 6. A mould according to claim 5, wherein there isa stepwise transition between the columnar wall and the end surface suchthat the columnar wall connects to a ring-shaped surface at a distanceof said end surface whereby a further columnar wall having a smallercross-section than the cross-section of the first mentioned columnarwall is present between said ring-shaped surface and the end surface. 7.A mould according to claim 6, wherein the stepwise transition isrealized by making the component out of two parts, i.e. acolumnar-shaped part determining the end surface and a tube, fittingaround said columnar-shaped part and determining the ring-shapedsurface.